1. Field of the Invention
This invention relates to bandgap voltage references, and more specifically to bandgap voltage reference circuits with high power supply ripple rejection.
2. Description of the Related Art
In the design of various analog circuits, such as digital to analog converters, voltage regulators, or low drift amplifiers, it is necessary to establish an independent bias reference within the circuit. This stable bias reference can be either a current or a voltage. In most applications, voltage rather than current references are preferred since they are easier to interface with the rest of the circuitry. Voltage references are required to provide a substantially constant output voltage regardless of changes in input voltage, output current, or temperature.
Temperature-compensated bias references are described in a number of publications, including an article by Paul Brokaw, "A Simple Three-Terminal IC Bandgap Reference," IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 6, December 1974, PP 388-393, and in Grebene, Bipolar and MOS Analog Integrated Circuit Design, John Wiley & Sons, 1984 PP 204-209. In designing temperature-compensated bias references, one starts with a predictable temperature drift, and then finds another predictable temperature source with temperature drift in the opposite direction that can be scaled by a temperature independent scale factor. Then, by proper circuit design, the effects of the two opposite-polarity drifts are made to cancel, resulting in a nominally zero temperature coefficient voltage level.
Three basic temperature drift sources exist that are reasonably predictable and repeatable. The first is the temperature dependence of a bipolar transistor base-emitter voltage drop VBE that exhibits a strong negative temperature coefficient, typically about -2 mV/.degree. C. The second is the temperature dependence of the V.sub.BE difference .DELTA.V.sub.BE between two transistors, which is proportional to absolute temperature through the thermal voltage V.sub.T and thus exhibits a positive temperature coefficient. The third and last temperature drift source is that of the base-emitter voltage of a Zener diode V.sub.Z, which is inherently low and positive in polarity.
By scaling one or more of these drift sources and subtracting them from each other, one may achieve the required compensation to provide a temperature independent bias voltage. Most voltage references are generally based on either Zener diodes or bandgap generated voltages. Zener devices characteristically exhibit high power dissipation and poor noise specifications. Bandgap generated voltage references compensate the negative V.sub.BE temperature drift with the positive thermal voltage temperature coefficient of V.sub.T, with V.sub.T equal to kT/q, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin and q is the electron charge.
In a simplified model, the output reference voltage V.sub.out may be expressed as: EQU V.sub.out =V.sub.BE +KV.sub.T. (1)
Since the two terms in the above equation exhibit opposite-polarity temperature drifts, it should be possible, at least in theory, to make V.sub.out nominally independent of temperature. A temperature-stabilized output dc level, in which .differential.V.sub.out /.differential.T is nominally equal to zero, is realized at an output voltage level on the order of +1.25V, which is very near the bandgap voltage of silicon. The name bandgap reference is derived from this relationship. Numerous variations in the bandgap reference circuitry have been designed, and are discussed for example in the U.S. Pat. Nos. 5,352,973 and 5,291,122 to Audy, assigned to Analog Devices, Inc., the assignee of the present invention.
A voltage reference, in addition to being temperature independent, also ideally applies a substantially constant output voltage irrespective of changes in input voltage supply or output current. These changes create signal noise (ripple) which degrades the overall stability of the voltage output, and should be rejected. The degree of rejection is called the high power supply ripple rejection (PSR). Known bandgap references generally fail to supply a substantially constant output reference voltage.